The present invention relates to a sheet-supply device for supplying cut sheets and to an image forming device provided with the sheet-supply device.
Recently, image forming device such as laser printers, color ink jet printers, facsimile machines, and copy machines, are provided with a sheet-supply device that supplies one cut sheet at a time to an image forming section of the image forming device. U.S. Pat. No. 6,158,733 and Japanese Patent Application Publication Nos. 2001-253580 and 2000-168980 disclose sheet-supply devices that include a slanting tray plate, a separation plate, and a sheet-supply roller. A plurality of sheets is stacked on the tray plate. The sheet-supply roller is provided in confrontation with the tray plate and supplies sheets downstream in a sheet-supply direction. The separation plate is disposed downstream from the tray plate in the sheet-supply direction. The separation plate has a separation slanted surface that extends in a direction that forms an obtuse angle with respect to the surface of the tray plate.
In one known sheet-supply device, the separation plate is modified such that the separation plate is formed with a slot extending in the direction in which the separation slanted surface extends. A high-friction insert member provided with a protruding part is fitted into the slot from the under side of the separation plate so that the protruding part is directed upwardly. The high-friction insert member is made from polyurethane resin. A foam-rubber member is provided at a lower surface of the high friction insert member for resiliently supporting the high-friction insert member.
The sheets stacked on the tray plate are supported with their leading edges, i.e., downstream-side (with respect to the direction of sheet supply) edges in abutment with the protruding part that protrudes from the surface of the separation plate. When the sheet-supply roller is driven to rotate while pressed against the uppermost sheet in the stack on the tray plate, then the leading edge of the sheet presses against the protruding part of the high-friction insert member. Therefore, the protruding part is resiliently deformed and sinks into the slot from the surface of the separation plate. As a result, the leading edge of the sheet is released from the high friction resistance of the high-friction insert member and so slides across the surface of a separation plate having a coefficient of friction lower than that of the high friction insert member, so that one sheet at a time is separated from the stack.
However, the separation plate is made completely from a synthetic resin in a block shape. Therefore, changes in temperature, humidity, or other environmental conditions, or in forming conditions, may change Young's modulus and friction coefficient of the synthetic resin. Thus, sheet separation ability varies with the season of the year, so that sometimes two sheets are fed out at the same time.